RESONANT OBLIQUITY OF MARS

The obliquity of Mars executes large-scale oscillations as a result of spin axis and orbit plane precessions. This motion is an important driver of the climate system and exhibits two dominant periodicities: (i) oscillations driven by the differential spin axis and orbit precession rates during which the obliquity may change by ≳20° over ∼105 years and (ii) amplitude modulations over ∼106 years due to changes in the orbital inclination. This paper discusses a third time scale for change involving a possible spin-orbit resonance. New work on secular orbit theory implies that Mars may have passed through such a resonance recently. The mechanism typically operates on a 107-year time scale and suggests an increase in the average obliquity of Mars of nearly 15° only ∼5 × 106 years ago. This could have important implications for the planet's climatic history. © 1991.